This invention relates to aluminum alloy electrical conductors. Examples of such conductors are wire, transformer strip, and the like.
Commercial purity aluminum has been employed as an electrical conductor for a long period and is increasingly replacing copper for that purpose. For reasons of economy, it is always necessary to employ a grade of aluminum having as high a conductivity as possible, consistent with the mechanical strength necessary to perform its function and also consistent with cost.
While very pure grades of aluminum may have a conductivity of 63.5 - 65% of the conductivity of pure annealed copper, the mechanical strength of the metal is unacceptably low. Electrical conductor (E.C.) grade aluminum commonly has a conductivity of 61.0 - 63.5% of the conductivity of copper and is a commercial purity aluminum generally containing 0.15 - 0.40% Fe and below 0.10% Si as the main impurities. Other impurities are maintained at a low value. An illustrative specification for E.C. grade aluminum alloy is as follows: less than 0.04% Cu, 0.15 - 0.40% Fe, 0.10% max. Si, 0.02% max. Zn, 0.02% max B, 0.02% max. Ga, and other elements less than 0.010% each, with a minimum Al content of 99.45%. It will be understood that all percentages set forth herein are expressed as percent by weight unless otherwise specifically indicated.
Increase in the levels of impurities and alloying elements leads to reductions in the conductivity of the metal.
The major obstacle to the use of aluminum wire for general electrical purposes is the uncertainty and difficulty of making stable, low-resistance connections. The surface of aluminum is normally protected by a film of oxide which is highly refractory and electrically insulating. Where this film remains between the contact faces, it increases the electrical resistance of the connections. Moreover, the aluminum alloys in common use for conductor wire, being close to the pure metal, soften relatively rapidly with rise in temperature and have poor creep resistance. Because of this, even where an adequate contact is established initially, it may not remain so, because of relaxation of the contact pressure, and the contact resistance will tend to increase for this reason.
When a current is passed through a contact having inadequate contact pressure, heat energy is liberated, chiefly at the points of high resistance at the contact faces, and causes a rise in temperature. This causes thermal expansion to take place, chiefly in the aluminum wire. It also produces softening of the wire and increase in the creep, with corresponding reduction in contact pressure and still further increase in resistance.
This process is cumulative and even if the contact faces are clean and free from oxide initially, at some stage it is possible for air to gain access to the contact faces and cause oxidation, particularly under intermittent operation, resulting in rapid and complete failure of the connection.
In order to provide an aluminum wire which is less subject to the above-indicated difficulties and thus rendered more suitable for general electrical wiring purposes, it is necessary both to increase its creep resistance and also to increase its thermal stability, i.e. to reduce its tendency to soften with slight temperature rise, such as typically occurs in connections in domestic electrical wiring.
Bearing in mind the importance of maintaining the conductivity of the aluminum at as high a level as possible, alloying additions for the purpose of increasing creep resistance and reducing the tendency of the metal to soften with temperature increase are maintained at as low a level as is consistent with these objectives.
For many purposes, the minimum acceptable conductivity of aluminum conductors is 60% of the International Annealed Copper Standard (IACS). Since alloying additions, especially such as might provide improvement with respect to creep and softening, generally tend to reduce conductivity, the difficulty of achieving a conductor that possesses both adequate conductivity and satisfactory creep resistance and thermal stability has heretofore limited use of aluminum conductors.